The present disclosure relates to transistors and methods for fabricating the same, and specifically to a transistor including a nitride semiconductor and a method for fabricating the same.
Group III nitride semiconductors (hereinafter abbreviated to nitride semiconductors), such as gallium nitride (GaN), have wide band gaps and large breakdown electric fields. Thus, the nitride semiconductors are expected as new materials which are used in high-output transistors and have advantages over silicon (Si) and gallium arsenide (GaAs). The band gaps of the nitride semiconductors can be freely changed by changing a mixed crystal ratio. An AlGaN/GaN heterostructure is formed by an aluminum gallium nitride (AlGaN) layer and a gallium nitride (GaN) layer having different band gaps. In the AlGaN/GaN heterostructure, a charge is generated at a heterojunction on a (0001) plane of a crystal structure due to spontaneous polarization and piezo-polarization. Thus, even when layers are undoped, a sheet carrier density of 1×1013 cm−2 or higher can be obtained. Therefore, a heterojunction field effect transistor (HFET) using a charge generated at a heterojunction as a channel can achieve a high current density to increase output power.
There is a demand for increasing a breakdown voltage and output power of the HFET. In order to increase the breakdown voltage of the HFET, an underlying layer provided between a layer (for example, an operation layer) in which electrons or positive holes serving as carriers are transported and a substrate has to have high insulating properties. In a method for improving the insulating properties of the underlying layer, doping with a dopant for trapping carriers is performed in crystal growth of the underlying layer. As the dopant, iron (Fe), which is a transition metal, is generally used (see, for example, Japanese Translation of PCT International Application No. 2009-519202, Japanese Translation of PCT International Application No. 2007-534580, Japanese Patent Publication No. 2007-184379, and Sten Heikman, et al., Applied Physics Letters, Vol. 81, p. 439 (2002)).